Abstract:

An object of the present invention is to stabilize the properties of
nanofibers produced.
Solution prepared by dissolving a polymeric substance in a solvent is
supplied into a conductive ejection container having a plurality of
ejection holes. The ejection container is rotated and electrostatic
explosions of the solution discharged through the ejection holes are
caused so that nanofibers are produced. In the above method for producing
nanofibers, in the case where the amount of the solution contained in the
ejection container exceeds a predetermined amount, the amount of the
solution exceeding the predetermined amount overflow the ejection
container. The overflowed solution is collected and resupplied to the
ejection container.

Claims:

1. A nanofiber producing method including: supplying solution which is raw
material liquid into an ejection container which is conductive and has a
plurality of ejection holes, the raw material liquid being prepared by
dissolving a polymeric substance in a solvent; and rotating the ejection
container so that the solution discharged through the plurality of
ejection holes is electrostatically exploded, said nanofiber producing
method comprising, in the case where an amount of the solution contained
in the ejection container exceeds a predetermined amount:allowing an
amount of the solution exceeding the predetermined amount to overflow the
ejection container;collecting the solution which has overflowed;
andresupplying the solution which has been collected to the ejection
container.

2. The nanofiber producing method according to claim 1,wherein the
ejection container is a cylindrical container and has the plurality of
ejection holes on a circumferential wall, andsaid method further
comprisesallowing the amount of the solution exceeding the predetermined
amount to overflow through a weir which has an annular shape and is
provided at one end of the ejection container.

3. A nanofiber producing apparatus comprising:an ejection unit configured
to eject solution which is raw material liquid for nanofibers; anda
charging unit configured to charge the solution by applying an electric
charge to the solution,wherein said ejection unit includes:an ejection
container which has a cylindrical shape with an ejection hole on a
circumferential wall and ejects the solution contained inside by a
centrifugal force caused by rotation of said ejection container;a
solution storage unit configured to store the solution to be transported
to said ejection container, and to store the solution which has
overflowed said ejection container; anda transporting unit configured to
transport the solution from said solution storage unit to said ejection
container.

4. The nanofiber producing apparatus according to claim 3,wherein said
ejection container includes a weir which has an annular shape and is
provided on an inner circumferential surface of an end of said ejection
container, said weir projecting inward said ejection container.

5. The nanofiber producing apparatus according to claim 3, further
comprisinga gas flow generating unit provided at a distance from said
ejection container in an axial direction of said ejection container.

6. The nanofiber producing apparatus according to claim 5, further
comprisinga windshield case, inside which said solution storage unit and
said transporting unit can be provided, and which prevents gas flow
generated by said gas flow generating unit from flowing into inside of
said windshield case.

7. The nanofiber producing apparatus according to claim 5, further
comprisinga guiding body which guides gas flow for transporting the
solution which has been ejected or the nanofibers which have been
produced,wherein said solution storage unit is provided inside said
guiding body.

8. The nanofiber producing apparatus according to claim 3,wherein said
ejection unit includes a case for holding said ejection container which
rotates, andsaid solution storage unit is provided inside said case.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a method and an apparatus for
producing nanofibers made of polymeric substances.

BACKGROUND ART

[0002]Conventionally, electrospinning (electric charge induced spinning)
is known as a method for producing filamentous (fibrous form) substances
(nanofibers) made of polymeric substances and such and having a diameter
in a submicron order.

[0003]In the electrospinning method, nanofibers are produced by ejecting
(discharging), to a space, solution which is raw material liquid prepared
by dispersing or dissolving polymeric substances and such in solvent,
applying an electric charge to the solution for charging, and allowing
the solution traveling in the space to be electrostatically exploded.

[0004]More specifically, as the solvent evaporates from particles of the
solution traveling the space, volume of the solution decreases. On the
other hand, the electric charge applied to the solution remains, which
results in increasing charge density of the particles of the solution.
Since the solvent continuously evaporates, the charge density of the
particles of the solution further increases. When Coulomb force, which is
generated in the solution particles and acts oppositely, exceeds the
surface tension of the solution, polymer solution undergoes a phenomenon
in which the polymer solution is explosively stretched into filament
(electrostatic explosion). Such electrostatic explosion is repeatedly
generated in the space, thereby producing nanofibers made of polymers
with a submicron diameter (for example, see patent reference 1).

[0005]By depositing thus produced nanofibers on a substrate or the like, a
thin film having 3-D structure of 3-D mesh can be obtained. Further, by
depositing the nanofibers thicker, a highly porous web having submicron
mesh can be produced. Thus produced thin film and highly porous web can
be preferably applied to a filter, a separator for use in a battery, a
polymer electrolyte membrane or an electrode for use in a fuel cell, or
the like. Such applications of the highly porous web made of the
nanofibers are expected to significantly improve performances of those
devices.

[0006]However, since, in the conventional electrospinning method, only a
small amount of nanofibers can be produced from the tip of a single
nozzle, the productivity of nanofibers cannot be improved. Consequently,
as a method for producing a large amount of nanofibers, a method
utilizing a plurality of nozzles has been proposed (for example, see
patent reference 2).

[0007]With reference to FIG. 1, the structure of an apparatus for
producing a polymeric web described in the patent reference 2 is
described as follows. A liquid polymeric substance in a barrel 43 is
supplied to a spinning unit 42 having a plurality of nozzles 41 by a pump
44. A high voltage of from 5 to 50 kV is applied to the nozzles 41 by a
high voltage generating unit 45. Fibers discharged through the nozzles 41
are deposited on a collector 46 that is either grounded or charged to a
polarity different from that of the nozzles 41 to form a web. The formed
web is transported by the collector 46, and a polymeric web is produced,
accordingly. It is also described in the reference that a charge
distributor 47 is provided in the vicinity of the tips of the nozzles 41
to minimize electrical interference among the nozzles 41 and that a high
voltage is applied to between the charge distributor 47 and the collector
46 so that an electric field which urges the charged fibers towards the
collector 46 is created.

[0008]Furthermore, as shown in FIGS. 2 (a) and (b), it is also described
in the reference that, instead of providing a plurality of single
nozzles, a plurality of multi-nozzles 41A, each including a plurality of
nozzles 41, are provided to the spinning unit 42 such that a plurality of
nanofibers are produced from each of the multi-nozzles 41A.

[0009]In order to produce the nanofibers with an improved productivity
using the structure shown in FIG. 1 and FIG. 2, it is conceivable that
the nozzles 41 in the spinning unit 42 or the nozzles 41 in each
multi-nozzle 41A are provided at smaller intervals so that the number of
nozzles per unit area is increased. In this case, however, as shown in
FIG. 3, the polymeric substances discharged through each nozzle 41 repel
each other as indicated by arrows F since the polymeric substances are
charged of the same polarity. Consequently, the discharge from the
nozzles 41 located in the middle is hampered. Further to this, the
discharge from the nozzles 41 located at a peripheral area is directed
outward. As a result, the deposition distribution of nanofibers on the
collector 46 becomes extremely sparse at the central area and
concentrated at the peripheral area, thereby failing to produce a uniform
polymeric web.

[0010]If the charge distributor 47 is provided in the vicinity of the tips
of the nozzles 41, electrical interference among the nozzles 41 is
reduced as shown in FIG. 4. In addition to this, the polymeric substances
discharged through each of the nozzles 41 is accelerated toward the
collector 46 because an electric field E from the charge distributor 47
to the collector 46 is created. As a result, as compared to the case of
FIG. 3, the deposition distribution of nanofibers at the central area and
at the peripheral area can be uniformed to a certain extent. However, at
the same time, the disposition pattern of the nozzles 41 is directly
reflected in the deposition distribution. Therefore, the above-mentioned
arrangement is not sufficiently effective in uniforming the deposition
distribution.

[0011]Furthermore, if provision density of the nozzles 41 is raised,
fibers may come to be in contact each other and stick together without
sufficiently evaporating the solvent. In addition to this, the
concentration of the evaporated solvent may increase in the vicinity of
the nozzles so that the insulation weakens, and accordingly, corona
discharge takes place, thereby failing to form fibers.

[0012]Furthermore, if a number of nozzles 41 are to be provided, it is
difficult to supply a liquid polymeric substance evenly to each of the
nozzles 41. This may complicate the structure of the apparatus and raise
the cost of facility. In addition to this, in order to cause an
electrostatic explosion of the liquid polymeric substance discharged
through the nozzles 41, the electric charge needs to be concentrated,
and, accordingly, each of the nozzles 41 is formed in a long and narrow
shape. However, it is also extremely difficult to conduct the maintenance
on a number of long and narrow nozzles 41 in order to ensure that they
are constantly in a proper condition.

[0013]Thus, the applicant of the present invention previously proposed the
following structure (see patent reference: Japanese Patent Application
No. 2006-317003). As shown in FIG. 5, a rotary tube 53 is fixed coaxially
to one end of a conductive cylindrical container 51 having a plurality of
ejection holes 52 on its circumferential surface, such that the rotary
tube 53 is pivotally supported. A solution supplying unit 54 supplies
solution as raw material liquid 50 into an ejection container 51 through
a solution supply tube 55 inserted to the rotary tube 53. Then, the
rotary tube 53 is driven to rotate so as to rotate the rotary container
51, and the ejection container 51 is charged by a first high voltage
generating unit 56. As a result, filamentous solution discharged through
the ejection holes 52 are stretched by centrifugal force and an
electrostatic explosion induced by evaporation of solvent, thereby
producing nanofibers made of polymeric substances. In addition, a voltage
with a polarity identical to that of the ejection container 51 is applied
by a reflecting power source 58 to a reflecting electrode 57 provided at
a certain distance from one end of the axial direction of the ejection
container 51, so that the produced nanofibers are deflected and travel
toward the other end of the axial direction of the ejection container 51.
Then, a voltage with an electric potential different from that of the
charge of the ejection container 51 is applied, by a third high voltage
generating unit 60, to a conductive collector 59 provided at a certain
distance from the other side of the axial direction of the ejection
container 51, so that the nanofibers are deposited on the collector 59.

[0014]However, the following problem has been found in the structure shown
in FIG. 5. In order to continuously maintain stable centrifugal force
that acts on the solution 50 extruded through the ejection holes 52 of
the cylindrical container 51 and to discharge the solution 50 as
filaments evenly for producing uniform nanofibers, it is necessary to
detect the amount of the solution 50 contained in the ejection container
51 and to precisely control the solution supplying unit 54 such that an
almost constant amount of the solution 51 is always contained in the
ejection container 51. This results in complicating the structure, and
raising required costs.

[0015]The present invention is to solve the conventional problems
described above, and has an object to provide a method and an apparatus
for producing uniform nanofibers with high productivity using a simple
structure.

Means to Solve the Problems

[0016]A nanofiber producing method according to an aspect of the present
invention is a nanofiber producing method including: supplying solution
which is raw material liquid into an ejection container which is
conductive and has a plurality of ejection holes, the raw material liquid
being prepared by dissolving a polymeric substance in a solvent; and
rotating the ejection container so that the solution discharged through
the plurality of ejection holes is electrostatically exploded, the
nanofiber producing method comprising, in the case where an amount of the
solution contained in the ejection container exceeds a predetermined
amount: allowing an amount of the solution exceeding the predetermined
amount to overflow the ejection container; collecting the solution which
has overflowed; and resupplying the solution which has been collected to
the ejection container.

[0017]It should be noted that, in the present invention, in order to apply
an electric field to the filamentous solution discharged through the
ejection holes of the ejection container, a large potential difference is
applied between the ejection container and an object or a member that
constitutes a space for producing nanofibers. For example, when such an
object or a member that constitutes a space for producing nanofibers is
either the earth or a member such as the collector grounded to the earth,
a positive or negative high voltage with reference to the ground
potential is applied to the ejection container. When a high voltage that
is either positive or negative with reference to the ground potential is
applied to a member such as the collector that constitutes a space for
producing nanofibers, the ejection container may be grounded. The
ejection holes are not limited to those directly punched through the
circumferential wall of the ejection container. Needless to say, the
ejection holes may be provided by nozzles installed on the
circumferential wall of the ejection container.

[0018]According to the structure described above, the solution is
discharged through the ejection holes of the ejection container under the
influence of the centrifugal force and is electrically charged. At this
time, the solution stably discharged through the ejection holes is
stretched under the influence of the centrifugal force.

[0019]Further, electrical interference hardly occurs by rotating the
ejection container. This is because the solution discharged through the
adjacent ejection holes by the centrifugal force, travel not in parallel
to each other, but radially. More specifically, the solution with same
polarity travel gradually departing each other, which results in hardly
causing electrical interference.

[0020]As described, since electrical interference does not affect the
condition, the solution can be stretched reliably and effectively even if
the ejection holes are densely provided.

[0021]Then, the diameter of the stretched solution decreases due to the
evaporation of the solvent, and the charge density increases. At the time
when Coulomb force exceeds the surface tension, a primary electrostatic
explosion takes place in the solution. Then the polymer solution is
further stretched. As the evaporation of the solvent proceeds further, a
secondary electrostatic explosion takes place in a similar manner and the
polymer solution is explosively stretched. A tertiary electrostatic
explosion may take place, depending on the situation.

[0022]Accordingly, a large amount of nanofibers made of polymeric
substances and having a submicron diameter can be efficiently produced
from solution discharged as filaments through a plurality of ejection
holes, using a simple and compact structure.

[0023]Furthermore, since the solution discharged through the ejection
holes is first stretched by the centrifugal force, those small holes need
not be made to be extremely small. In addition, the ejection holes do not
need to be made of a long shape to concentrate the charges as described
above. Thus, it is only necessary that the ejection container be simply
provided with ejection holes. Hence, the ejection container can be
fabricated easily and at low costs. The maintenance can still be
conducted easily even though the ejection container is provided with a
large number of ejection holes.

[0024]Furthermore, an amount of the solution exceeding a predetermined
amount in the ejection container overflows the ejection container. Thus,
a constant amount of solution can be continuously maintained in the
ejection container by simply supplying sufficient amount of solution to
the ejection container. Accordingly, it is possible to maintain constant
centrifugal force acting on the solution extruded through the ejection
holes of the ejection container, thereby reliably producing uniform
nanofibers.

[0025]Furthermore, the solution overflowed the ejection container is
collected and transported to the ejection container again so that the
overflowed solution can be reused. As a result, the solution is not
unnecessarily consumed.

[0026]With above, production of uniform nanofibers with high productivity
is possible while realizing a cost reduction associated with apparatuses
and materials.

[0027]Further, it may be that the ejection container is a cylindrical
container and has the plurality of ejection holes on a circumferential
wall, and the method further includes allowing the amount of the solution
exceeding the predetermined amount to overflow through a weir which has
an annular shape and is provided at one end of the ejection container.

[0028]By having the ejection container which is a cylindrical container
and has the plurality of ejection holes on a circumferential wall, and
allowing the amount of the solution exceeding the predetermined amount to
overflow through a weir which has an annular shape and is provided at one
end of the ejection container, a large amount of nanofibers can be
uniformly produced from the whole circumference of the cylindrical
container at once, which secures high productivity. In addition, by
allowing the amount of the solution exceeding the predetermined amount to
overflow through a weir which has an annular shape and is provided at one
end of the ejection container, the above advantageous effects can be
obtained with an extremely simple structure.

[0029]On the other hand, in order to achieve the above object, the
nanofiber producing apparatus according to an aspect of the present
invention is a nanofiber producing apparatus including: an ejection unit
which ejects solution which is raw material liquid for nanofibers; and a
charging unit which charges the solution by applying an electric charge
to the solution, in which the ejection unit includes: an ejection
container which has a cylindrical shape with an ejection hole on a
circumferential wall and ejects the solution contained inside by a
centrifugal force caused by rotation of the ejection container; a
solution storage unit which stores the solution to be transported to the
ejection container, and to store the solution which has overflowed the
ejection container; and a transporting unit which transports the solution
from the solution storage unit to the ejection container.

[0030]With this structure, an amount of the solution exceeding a
predetermined amount in the ejection container overflows the ejection
container. Thus, by simply supplying sufficient amount of solution to the
ejection container, constant amount of solution can be continuously
maintained in the ejection container, and constant centrifugal force
which acts on the solution contained in the ejection container can also
maintained. Accordingly, uniform nanofibers can constantly be produced.

[0031]Further, since the overflowed solution can be collected and reused,
the solution is not unnecessarily consumed. This allows production of
uniform nanofibers with high productivity while realizing a cost
reduction associated with apparatuses and materials.

[0032]It is preferable that the ejection container includes a weir which
has an annular shape and is provided on an inner circumferential surface
of an end of the ejection container, the weir projecting inward the
ejection container.

[0033]Further, by having the ejection container which is a cylindrical
container and has the plurality of ejection holes on a circumferential
wall, it is possible to uniformly produce a large amount of nanofibers
from the whole circumference of the cylindrical container at once, which
secures high productivity. Further, at one end of the ejection container,
an annular weir through which the amount of the solution exceeding the
predetermined amount to overflow, is provided. With this, when the
solution is supplied to the ejection container exceeding the
predetermined amount, the exceeding amount of the solution overflows
through the annular weir provided at one end of the ejection container.
In other words, by simply supplying a sufficient amount of the solution,
a constant and desired amount of the solution can be maintained in the
ejection container. Accordingly, the centrifugal force acts on the
solution extruded through the ejection holes of the ejection container
can be maintained at a desired value, thereby controlling the quality of
the nanofibers to a certain extent.

[0034]Further, a gas flow generating unit may also be provided at a
distance from the ejection container in an axial direction of the
ejection container.

[0035]With this, blowing from one end of the axial direction of the
ejection container allows effective deflection of the direction of travel
of the nanofibers which are being produced. Further, evaporated solvents
are moved out of the manufacturing space immediately, which results in
not increasing solvent concentration in the surrounding atmosphere. This
facilitates solvent evaporation, and reliably produces effects of the
electrostatic explosion, thereby reliably producing desired nanofibers.

[0036]Further, the nanofiber producing apparatus may also include a
windshield case, inside which the solution storage unit and the
transporting unit can be provided, and which prevents gas flow generated
by the gas flow generating unit from flowing into inside of said
windshield case.

[0037]With this, it is possible to transport nanofibers and the like with
gas flow. This also allows the nanofibers and the like to be collected by
gas flow. As a result, it is possible to collect the nanofibers with high
density. Further, the windshield case isolates circulating solution from
the gas flow; and thus, solvent evaporation is not easily accelerated by
the gas flow. Thus, it is possible to obtain the stable quality of the
solution. Further, in this case, the solution storage unit and the
transporting unit are provided near the ejection container; and thus,
minimizing degradation of the solution is possible.

EFFECTS OF THE INVENTION

[0038]According to a nanofiber producing method and a nanofiber producing
apparatus of the present invention, a large amount of nanofibers made of
polymeric substances and having a submicron diameter can be efficiently
produced from solution as raw material liquid discharged through a
plurality of ejection holes, using a simple and compact structure.
Furthermore, since the solution discharged through the ejection holes is
first stretched by the centrifugal force, the ejection holes need not be
made to be extremely small. The ejection holes do not also need to be
made of a long shape to concentrate the charges as described above. Thus,
it is only necessary that the ejection container be simply provided with
ejection holes. Accordingly, the ejection container can be fabricated
easily and at low costs, and the maintenance can be conducted easily even
though the ejection container is provided with a large number of ejection
holes.

[0039]Furthermore, the amount of the solution exceeding a predetermined
amount in the ejection container overflows the ejection container. Thus,
a constant amount of solution can be continuously maintained in the
ejection container by simply supplying sufficient amount of solution to
the ejection container. Accordingly, it is possible to maintain constant
centrifugal force that acts on the solution extruded through the ejection
holes of the ejection container, thereby constantly producing uniform
nanofibers. In addition, the overflowed solution is collected and
resupplied to the ejection container, so that the overflowed solution is
reused. This prevents the solution from being unnecessarily consumed.
Therefore, production of uniform nanofibers with high productivity is
possible while realizing a cost reduction associated with apparatuses and
materials.

[0040]Further, according to the present invention, the solution circulates
between the ejection container and the solution storage unit provided
near the ejection container. This prevents the degradation of the
solution due to circulation of the solution. Therefore, it is possible to
obtain stable quality of nanofibers produced from the solution.

BRIEF DESCRIPTION OF DRAWINGS

[0041]FIG. 1 is a schematic structure of an apparatus for producing a
polymeric web according to a conventional example.

[0042]FIG. 2 shows essential parts of another example of a structure of
the conventional example, (a) being a front view, and (b) being a
partially enlarged bottom view.

[0043]FIG. 3 is a diagram illustrating problems faced in the conventional
example.

[0044]FIG. 4 is a diagram illustrating still other problems faced in the
conventional example.

[0045]FIG. 5 is a longitudinal front view of a nanofiber producing
apparatus disclosed prior to the present invention.

[0046]FIG. 6 is a longitudinal front view of a nanofiber producing
apparatus according to a first embodiment of the present invention.

[0047]FIG. 7 is a perspective view of a producing state of a polymeric web
according to the first embodiment of the present invention.

[0048]FIG. 8 is a block diagram showing a control structure according to
the first embodiment of the present invention.

[0049]FIG. 9 is a longitudinal front view of a nanofiber producing
apparatus according to a second embodiment of the present invention.

[0050]FIG. 10 is a perspective view of a producing state of a polymeric
web according to the second embodiment of the present invention.

[0051]FIG. 11 is a longitudinal front view of a nanofiber producing
apparatus according to a third embodiment of the present invention.

[0052]FIG. 12 is a cross sectional view schematically showing a nonwoven
fabric producing apparatus according to an embodiment of the present
invention.

[0053]FIG. 13 is a cross-sectional view of a raw material discharging
unit.

[0054]FIG. 14 is a perspective view of an appearance of an ejection unit.

[0055]FIG. 15 is a cross-sectional view of a variation of the ejection
unit.

[0056]FIG. 16 is a cross-sectional view of another variation of the
ejection unit.

[0126]Hereinafter, embodiments of a nanofiber producing method and a
nanofiber producing apparatus according to the present invention will be
described with reference to the drawings.

First Embodiment

[0127]Firstly, a first embodiment of a nanofiber producing apparatus
according to the present invention will be described.

[0128]As shown in FIGS. 6 and 7, an ejection container 1 is a cylindrical
container having a diameter of 30 to 400 mm. The ejection container 1 is
integrally and coaxially fixed to a cylinder 2 such that one end of the
cylinder 2 penetrates one end of central axis of the ejection container
1. Accordingly, the ejection container 1 is pivotally supported about the
central axis by the cylinder 2 as shown by an arrow R. The cylinder 2 is
made of materials with high electric insulating properties. The other end
of the ejection container 1 is closed. A number of ejection holes 3 of
0.01 to 2 mm in diameter are provided on the circumferential surface of
the ejection container 1 at intervals of a few millimeters.

[0129]The ejection holes 3 may be formed by directly punching through the
circumferential wall of the ejection container 1; however, it may be that
short nozzles each having a hole serving as the ejection hole are
installed on the circumferential wall of the ejection container 1.

[0130]The cylinder 2 is pivotally supported via a bearing 5 by a support
frame 4 made of materials with high electric insulating properties. The
cylinder 2 is driven to rotate at a rate of 30 to 10000 rpm by a motor 9
serving as a rotation drive unit, via a belt 8 which is wound around
between a pulley 6 provided on the outer circumferential surface of the
cylinder 2 and a motor pulley 7 provided on the output axis of the motor
9.

[0131]A preferable motor to be used as the motor 9 is a sensorless DC
motor, because a sensor may improperly operate under influence of high
voltage noise.

[0132]A solution supply tube 10 as a transporting unit is inserted to the
ejection container 1 through the center of the cylinder 2. The tip of the
solution supply tube 10 is a discharging portion 10a which is a curved
L-shaped toward bottom. Solution 11 prepared by dissolving, in a solvent,
polymeric substances which are materials for nanofibers, is supplied into
the ejection container 1 via the solution supply tube 10. By supplying
the solution 11 into the ejection container 1 and rotating the ejection
container 1, the excessively supplied solution 11 overflows to the
outside through the cylinder 2 while the inner circumferential surface of
the cylinder acting as a weir. As a result, a layer of the solution 11
with even thickness is formed on the whole inner circumferential surface
of the ejection container 1. More particularly, where the inside diameter
of the ejection container 1 is D1, and the inside diameter of the
cylinder 2 is D2, a layer of the solution 11 with approximately uniform
thickness of T=(D1-D2)/2 is formed on the inner circumferential surface
of the ejection container 1.

[0133]A solution storage unit 12, which serves as a collecting unit, is
provided above the support frame 4, so that the solution 11 overflowed to
the outside through the cylinder 2 are collected. As indicated by a
virtual line in FIG. 6, a receiving unit 13 is provided which receives
the solution 11 overflowed the cylinder 2 and guides the solution 11 to
the solution storage unit 12 while preventing the solution from
dispersing. Further, an amount of the solution 11 equivalent to the
amount of the solution 11 which has been consumed is refilled to the
solution storage unit 12 by a solution refill unit (not shown). The
solution 11 in the solution storage unit 12 is suctioned through a
suction tube 14 by a transporting pump 15 serving as a transporting unit,
and is transported toward the ejection container 1 through the solution
supply tube 10 at a predetermined flow rate.

[0136]The solution can be mixed with an inorganic solid material, examples
of which include oxides, carbides, nitrides, borides, silicides,
fluorides, and sulfides. However, in terms of thermal stability,
workability, and the like, oxides are preferable. Examples of oxides
include Al2O3, SiO2, TiO2, Li2O, Na2O, MgO,
CaO, SrO, BaO, B2O3, P2O5, SnO2, ZrO2,
K2O, Cs2O, ZnO, Sb2O3, As2O3, CeO2,
V2O5, Cr2O3, MnO, Fe2O3, CoO, NiO,
Y2O3, Lu2O3, Yb2O3, HfO2, and
Nb2O5. Although at least one type selected from the above is
used, the present invention should not be limited thereto.

[0137]Desirable mixing ratio of solvent and polymeric substance is that
the polymeric substances constituting the nanofiber be selected in the
range of not less than 1 vol % and not more than 50 vol %, and the
corresponding solvent be selected in the range of not less than 50 vol %
and not more than 99 vol %.

[0138]A high voltage of 1 kV to 200 kV, preferably 10 kV to 100 kV,
generated by a charge power source 16 serving as a charging unit, is
applied to the ejection container 1 via the bearing 5, and a conductive
member 17. Accordingly, the solution 11 contained in the ejection
container 1 is also subject to this high voltage. As a method for
applying a high voltage, a high voltage may be applied to the ejection
container 1 by the charging unit via a slip ring or a brush.

[0139]When the ejection container 1 is driven to rotate at a high speed by
the mortor 9, centrifugal force acts on the solution 11. Then, the
solution 11 is discharged as filaments through each of the ejection holes
3. The filaments of the solution 11 are then stretched under the
influence of the centrifugal force, thereby producing thin filamentous
solution. The filamentous solution to which the high voltage is applied,
is then subjected to an electric field that is formed around the ejection
container 1, and is electrically charged. When the solvent of the
solution 11 evaporates, the diameter of the polymeric filament decreases.
With this, the density of the electric charge residing thereon becomes
concentrated. When Coulomb force exceeds the surface tension of the
solution 11, a primary electrostatic explosion takes place, and the
solution 11 is explosively stretched. Then, as the solvent further
evaporates, a secondary electrostatic explosion takes place, and the
solution 11 is further stretched explosively in a similar manner.
Depending on the condition, a tertiary electrostatic explosion and so on
may take place. Consequently, nanofibers f that have submicron diameters
and are made of polymeric substances are effectively produced.

[0140]A reflecting electrode 41 is provided to the support frame 4 so as
to be positioned directly opposite to one end of the ejection container 1
with a suitable distance. A high voltage generated by a reflecting power
source 19 is applied to the reflecting electrode 41. The reflecting power
source 19 generates a high voltage with the polarity identical to that of
charge power source 16 and approximately same level, and applies the
generated voltage to the reflecting electrode 41. As shown in FIG. 7, the
reflecting electrode 41 causes the filamentous solution produced by being
discharged from the ejection container 1 and stretched, and the
nanofibers f produced by the successively generated electrostatic
explosion, to travel toward the other end of the ejection container 1 as
indicated by the arrow D.

[0141]A conductive collector 20 is provided so as to be directly opposite
to the other end of the ejection container 1 with a suitable distance. A
high voltage, which is generated by a collector power source 21 and has a
polarity opposite to that of the voltage applied to the ejection
container 1, is applied to the collector 20.

[0142]Since it is only necessary that a large potential difference is
created between the ejection container 1 or the reflecting electrode 41
and the collector 20 so that an electric field is generated therebetween.
Thus, the collector 20 may be simply grounded.

[0143]With an electric field generated by a large potential difference
between the ejection container 1 or the reflecting electrode 41 and the
collector 20, the nanofibers f are produced as described above. Then, as
shown in FIG. 7, the charged nanofibers f are caused to travel toward the
collector 20 to be deposited thereon. By applying, to the collector 20, a
high voltage with a polarity opposite to that of the ejection container
1, it is possible to allow the produced nanofibers f to be deposited on
the collector 20 even when the ejection container 1 and the collector 20
are distant from each other by, for example, approximately 2 m.

[0144]It is preferable that the charge power source 16, the reflecting
power source 19, and the collector power source 21 are respectively
switched on and off as necessary by switches SW1, SW2, and SW3.

[0146]In the figure, a control unit 22 controls the motor 9, the
transporting pump 15, the charge power source 16, the reflecting power
source 19, and the collector power source 21. In accordance with an
operational instruction from an operation unit 23, the control unit 22
controls operations based on operation programs stored in a memory unit
24 or various kinds of data inputted by the operation unit 23 and stored,
and displays the operational status or various kinds of data onto a
display unit 25.

[0147]With the above structure, a predetermined amount of the solution 11
is supplied to the ejection container 1 by the transporting pump 15, and
a predetermined level of high voltage is applied to the ejection
container 1 by the charge power source 16, so as to charge the solution
11 contained in the ejection container 1 to a high voltage. By rotating
the ejection container 1 at high speed by the mortor 9 in such a state,
the solution is discharged as filaments through the ejection holes 3 to
become filamentous solution. The filamentous solution is then greatly
stretched under the influence of the centrifugal force. Then, the
filamentous solution charged to a high voltage is subjected to an
electric field and are electrically charged. When the filamentous
solution is further stretched making the diameter thereof decrease, and
the solvent evaporates, the electric charge becomes concentrated. As a
result, a primary electrostatic explosion takes place, thereby
explosively stretching the filamentous solution. Then, as the solvent
further evaporates, a secondary electrostatic explosion takes place, and
the filamentous solution is further stretched explosively in a similar
manner. Depending on the condition, a tertiary electrostatic explosion
and so on takes place, thereby causing further stretching. Accordingly,
the nanofibers f made of polymeric substances and having a submicron
diameter can be produced from filamentous solution discharged through the
ejection holes.

[0148]Here, a layer of the solution 11 with approximately uniform
thickness T is formed on the inner circumferential surface of the
ejection container 1. The solution 11 excessively supplied overflows to
the outside through the cylinder 2 acting as a weir, and is collected by
the solution storage unit 12 to be reused. As described, the amount of
the solution 11 in the ejection container 1 can be controlled to be
almost constant all the time; and thus, a constant centrifugal force acts
on the solution 11 in the ejection container 1, and centrifugal force
acts on the solution 11 discharged through the ejection holes 3 of the
ejection container 1 also becomes constant. As a result, the solution 11
can be evenly discharged as filaments, thereby producing uniform
nanofibers f.

[0149]Furthermore, when producing the nanofibers f, the filamentous
solution is stretched under the influence of the centrifugal force. The
direction of travel of the filamentous solution tends to be radial.
However, the reflecting electrode 41 deflects the direction toward the
other end of the axial direction of the ejection container 1. As a
result, the produced nanofibers f can be easily collected within a
predetermined area of the collector 20.

[0150]Furthermore, the reflecting electrode 41 is provided at a certain
distance from the one end of the ejection container 1; and thus, unlike
the case where the parabolic mirror type reflecting electrode 41 is
provided facing the outer circumferential surface of the ejection
container 1, the reflecting electrode 41 does not face the direction of
discharge of the charged solution 11. As a result, the electric charge of
the reflecting electrode 41 does not affect the discharge of the solution
11, thereby producing the nanofibers f stably and effectively.

[0151]Further, even if some solutions do not become fibers and remain as
droplets, such droplets disperse by the centrifugal force. Only
appropriate nanofibers f are deflected and travel toward the collector
20; and thus, only the nanofibers f with high quality can be collected.

[0152]Thus produced and electrically charged nanofibers f are deposited on
the collector 20. Accordingly, a highly porous polymeric web can be
produced with high productivity.

[0153]Further, since the solution filament, formed by being discharged
through the ejection holes 3 of the ejection container 1, is stretched
significantly by the centrifugal force, the ejection holes 3 can be made
to be approximately 0.01 to 2 mm in diameter. Therefore, the ejection
holes 3 do not need to be made extremely small. Furthermore, unlike the
case where the electrostatic explosion needs to take place first,
electric charge does not need to be concentrated; and thus, the ejection
holes 3 do not need to be formed as a long and narrow nozzle.
Furthermore, since the electric field interference does not affect the
situation, even when the ejection holes 3 are densely arranged, the
filamentous solution can be reliably and effectively stretched, thereby
effectively producing a large amount of nanofibers in a simple and
compact structure.

[0154]Furthermore, a large amount of nanofibers can be produced at a time
evenly from the entire circumferential surface of the ejection container
1, ensuring high productivity. Its simple shape and structure also
contribute to a cost reduction associated with production facilities.
Furthermore, the ejection holes 3 may be provided at the tips of the
nozzles. However, with the structure of the present invention, the
ejection holes 3 do not need to be made of a long and narrow shape; and
thus, these ejection holes 3 can be simply provided on the outer
circumferential surface of the ejection container 1. Hence, the ejection
container 1 can be fabricated easily and at low costs, and the
maintenance can be conducted easily even though the ejection container 1
is provided with a large number of ejection holes 3.

[0155]Further, the motor 9 is capable of controlling the rotation speed of
the ejection container 1 based on the viscosity of the solution 11
contained in the ejection container 1. This structure allows a required
centrifugal force to act on the solution 11 in accordance with the
viscosity of the solution 11, thereby reliably and effectively producing
nanofibers f.

[0156]Further, in the above drawings, an example has been shown where the
reflecting electrode 41 is fixed to the support frame 4 which is
insulated from the ejection container 1, and a high voltage generated by
the reflecting power source 19 is applied to the reflecting electrode 41.
However, it may be that the reflecting electrode 41 is fixed to the outer
circumferential surface of the cylinder 2, and is electrically connected
to the ejection container 1, so that a same level of high voltage
generated by the charge power source 16 is applied to the ejection
container 1 and the reflecting electrode 41. In this case, the reflecting
electrode 41 also rotates together with the ejection container 1, but
this does not impose any functional effects.

[0157]Furthermore, it may be that a blowing unit which blows toward the
other end of the ejection container 1 is provided between the reflecting
electrode 41 and the ejection container 1. Accordingly, evaporated
solvents are moved out of the manufacturing space immediately by blowing,
which results in not increasing solvent concentration in the surrounding
atmosphere. This facilitates solvent evaporation, and reliably produces
effects of the electrostatic explosion, thereby reliably producing
desired nanofibers f. Further, it also allows effective deflection of the
direction of travel of the nanofibers f being produced. Further, it may
be that instead of the reflecting electrode 41, a blowing unit which
blows gas toward the other end of the ejection container 1 is provided so
that the produced nanofibers can be deflected toward a desired direction.

[0158]Further, in the example of structure described above, the cylinder 2
can be driven to rotate, and the ejection container 1 is fixed to the
cylinder 2. However, it may be that the cylinder 2 is fixed to the
support frame 4, and the ejection container 1 is pivotally supported by
the cylinder 2. In this case, the receiving unit 13 is not necessary.

Second Embodiment

[0159]Next, second embodiment of a nanofiber producing apparatus according
to the present invention will be described with reference to FIG. 9 and
FIG. 10. In the following description of the embodiment, the same
elements as appeared in the preceding embodiment will be designated by
the same reference numerals, and descriptions of those elements will be
omitted while only differences will be described.

[0160]In the above first embodiment, an example has been shown where
solution 11 is suctioned by a transporting pump 15 through a suction tube
14 from a solution storage unit 12 provided above the support frame 4,
and is transported into an ejection container 1. In the present
embodiment, as shown in FIGS. 9 and 10, in addition to the solution
storage unit 12, a large volumetric storage container 26 is provided, and
the solution 11 collected into the solution storage unit 12 is
transported to the storage container 26 through a transporting tube 27.
The tip of the suction tube 14 connected to the suction inlet of the
transporting pump 15 is inserted into the storage container 26.

[0161]The present embodiment can also produce the effects similar to those
obtained in the first embodiment. In addition, since the excessive
solution 11 overflowed the ejection container 1 is collected into the
large volumetric storage container 26 via the solution storage unit 12
provided above the support frame 4, it is possible to stably supply the
solution 11 from the storage container 26 into the ejection container 1.

Third Embodiment

[0162]Next, third embodiment of a nanofiber producing apparatus according
to the present invention will be described with reference to FIG. 11.

[0163]In the present embodiment, an ejection container 1 includes a weir
31 at its one end. Further, the ejection container 1 has an opening at
the one end. A rotary shaft 32 penetrates the axial position of the
ejection container 1 through the opening at the one end toward the other
end of the ejection container 1 and is integrally connected to the closed
wall at the other end. The rotary shaft 32 is pivotally supported by a
shaft bearing 34 provided to a support cylinder 33 that is provided on a
support frame 4. The rotary shaft 32 has a tip connected via a shaft
coupling 32a to a motor 9 provided to the support cylinder 33, so that
the rotary shaft 32 can be driven to rotate by the motor 9.

[0164]At the outer circumference of one end of the support cylinder 33, a
receiving unit 13 is provided so as to surround the outer circumference
of the one end of the ejection container 1. The receiving unit 13 has a
return tube 13a for allowing the solution 11 collected inside the
receiving unit 13 to return to a solution storage unit 12.

[0165]The solution 11 in the solution storage unit 12 is supplied into the
ejection container 1 through a solution supply tube 10 by a transporting
pump 15 such as a gear pump. Further, a liquid surface sensor 35 is
provided for detecting a liquid surface level of the solution 11 in the
solution storage unit 12. When it is detected that a liquid surface level
is decreased to a certain level, the solution 11 is supplied by a refill
apparatus 36 such as a gear pump, from the storage container 26 to the
solution storage unit 12 through the supply tube 37 so that the liquid
surface level of the solution 11 in the solution storage unit 12 can be
maintained within an approximately constant range.

[0166]Further, a gas flow generating unit 38 is provided at a certain
distance from the support cylinder 33 which is a position opposite to the
ejection container 1. The nanofibers f, produced by being discharged from
the ejection container 1 and stretched, are caused to travel toward the
other end of the ejection container 1 by a gas flow W, which is generated
by the gas flow generating unit 38 and indicated by an arrow, instead of
causing the nanofibers f to travel toward the other end of the ejection
container 1 by an electric field generated by the reflecting electrode
41.

[0167]It may be that a wire mesh form reflecting electrode 41 is provided
around the outer circumferential surface of the support cylinder 33, such
that both an electric field generated by the reflecting electrode 41 and
the gas flow W cause the nanofibers f to travel toward the other end of
the ejection container 1.

[0168]The present embodiment also produces the effects similar to those
obtained in the first embodiment, and allows the compact structure of
rotation mechanism of the ejection container 1. In addition, the
excessive solution 11 in the ejection container 1 directly and smoothly
overflows over the weir 31 at the one end of the ejection container 1,
thereby maintaining constant thickness of the layer of the solution 11 in
the ejection container 1 with high responsiveness. Further, it is
possible to cause the nanofibers f produced from the ejection container 1
to smoothly travel toward the other end of the ejection container 1 by
the gas flow W generated by the gas flow generating unit 38.

[0169]In the description of the above embodiment, an example has been
described where either the reflecting electrode 41 or the gas flow
generating unit 38 is provided, or both of them are provided. However, it
may be that a collector 20, to which a high voltage with a polarity
opposite to that of a voltage applied to the ejection container 1 is
applied, or which is grounded, is simply provided so as to cause the
nanofibers f produced from the ejection container 1 to travel toward the
collector 20 and to deposit on the collector 20.

[0170]Furthermore, in the description of each embodiment above, an example
has been described where a high voltage is applied to the ejection
container 1 by the charge power source 16, and the collector 20 is
grounded, or a voltage with an opposite polarity is applied to the
collector 20 by the collector power source 21. However, it may be that a
positive or negative high voltage is applied to the collector 20 by the
collector power source 21 and the ejection container 1 is grounded.

Fourth Embodiment

[0171]Next, fourth embodiment according to the present invention is
described with reference to the drawings.

[0172]FIG. 12 is a cross sectional view schematically showing a nanofiber
producing apparatus according to an embodiment of the present invention.

[0173]As shown in FIG. 12, a nanofiber producing apparatus 100 includes a
nanofiber discharging apparatus 200, a collector 20, an attracting unit
102, an area control unit 103, a transporting unit 104, an attraction
control unit 105, and a solvent collect unit 106.

[0174]The collector 20 is a member, on which nanofibers f produced in the
air are deposited, and which has breathability so that the nanofibers f
transported by the gas flow are collected. In the present embodiment, the
collector 20 is a long sheet-shaped member which is thin and flexible,
and made of materials easily removable from the deposited nanofibers f.
More specifically, an example of the collector 20 is a long mesh made of
aramid fiber. Further, Teflon (registered trademark) coating on its
surface is preferable since it enhances removability of the collected
nanofibers f. The collector 20 is supplied being wound into a roll from a
supply roll 111.

[0175]The transporting unit 104 winds the long collector 20 and
simultaneously unwinds the long collector 20 from the supply roll 111,
and slowly moves the vicinity of the nanofiber discharging apparatus 200
so that the nanofibers f deposited on the collector 20 is transported.
The transporting unit 104 can wind the nanofibers f deposited in a
non-woven fabric like state, together with the collector 20.

[0176]The attracting unit 102 is provided at an opposite side of where the
nanofibers f are collected on the collector 20, that is an position side
of where the nanofiber discharging apparatus 200 is provided. The
attracting unit 102 is an apparatus which attracts gases forming the gas
flow traveled from the nanofiber discharging apparatus 200 through the
collector 20. In the present embodiment, the nanofiber producing
apparatus 100 includes a blower, such as a sirocco fan or an axial flow
fan, as the attracting unit 102. Further, the attracting unit 102 is
provided inside the duct 121. The attracting unit 102 is capable of
attracting the gas flow in which evaporated solvent is mixed, and also
transporting the gas flow to the solvent collecting apparatus 106 through
the duct 121.

[0177]The attraction control unit 105 is an apparatus which is
electrically connected to the attracting unit 102, and which controls the
attraction amount of the attracting unit 102. In the present embodiment,
a blower is used as the attracting unit 102. The attraction control unit
105 controls the attraction amount of gas by controlling the number of
rotation of the blower.

[0178]The area control unit 103 serves to control the attraction area of
the attracting unit 102, and is provided at an opposite side of where the
nanofibers f are collected on the collector 20, and provided between the
collector 20 and the attracting unit 102. The area control unit 103 is a
cylinder which has both ends which are opened. The area control unit 103
is preferably shaped such that it corresponds to the shape of the end of
the nanofiber discharging unit 200 which discharges the nanofibers f. For
example, when the end of the nanofiber discharging apparatus 200 has a
rectangular shape, it is preferably that the area control unit 103 be a
rectangular shaped cylinder. Further, when the end of the nanofiber
discharging apparatus 200 has a circular cylindrical shape, it is
preferable that the area control unit 103 also has a circular cylindrical
shape.

[0179]The nanofiber discharging apparatus 200 is an apparatus which ejects
the charged solution 11 into the air and produces the nanofibers f by
causing electrostatic explosion in the air. The nanofiber discharging
apparatus 200 includes an ejection unit 201, a charging unit 202, a
solution supplying unit 204, a gas flow generating unit 38, a heating
unit 205 and a guiding body 206.

[0180]Note that the raw material liquid to be used for producing the
nanofibers f is referred to as the solution 11, and the produced
nanofibers are referred to as the nanofibers f; however, the border
between the solution 11 and the nanofibers f is ambiguous; and thus, they
cannot be clearly distinguished from each other.

[0181]FIG. 13 is a cross-sectional view of a solution discharging unit.

[0182]Note that a solution discharging unit 290 is a collective term for
members such as the ejection unit 201, the charging unit 202, the gas
flow generating unit 38 (see below), the guiding body 206 (see below),
the heating unit 205 (see below) and the like.

[0183]The ejection unit 201 is an apparatus which ejects the solution 11
into the air, and includes an ejection container 1, a rotary shaft 32, a
motor 9, a solution storage unit 12, a transporting unit 215, and a
windshield case 216.

[0184]The ejection container 1 is a container which can eject (discharge)
the solution 11 into the air by the centrifugal force caused by rotation
of the ejection container while the solution 11 being supplied inside.
The ejection container 1 has a cylindrical shape whose one end is closed,
and includes a plurality of ejection holes 3 on its circumferential wall.
The ejection container 1 is formed of a conductive material so that an
electric charge can be applied to the solution 11 contained inside. The
ejection container 1 is pivotally supported by a bearing 5 provided to
the support body 262. More particularly, it is preferable that the
diameter of the ejection container 1 be set within a range of not less
than 10 mm to not more than 300 mm. It is because, if the diameter is too
large, causing the gas flow to concentrate the solution 11 or the
nanofibers f becomes difficult. In addition, in order to stably rotate
the ejection container, a stronger structure for supporting the ejection
container is required. On the other hand, if the diameter is too small,
it is necessary to increase rotation of the ejection container 1 so that
the solution 11 is ejected by the centrifugal force. This causes problems
associated with load of the motor, vibration or the like. Further, it is
preferable that the diameter of the ejection container 1 be set within a
range of not less than 20 mm to not more than 100 mm. Further, the
cross-sectional shape of the ejection hole 3 is a circle. The diameter of
the ejection hole 3 is preferably set within a range of not less than
0.01 mm to not more than 2 mm. However, the shape of the ejection hole 3
is not limited to circle, but may be polygonal, star like shape, or the
like.

[0185]At the other end of the ejection container 1, an annular weir 31 is
provided projecting inward from the circumference of the other end of the
ejection container 1. The weir 31 is a wall which acts as a weir for
storing a predetermined amount of the solution 11 inside the ejection
container 1. When the amount of the solution 11 exceeding the
predetermined amount is transported into the ejection container 1, the
solution 11 overflows over the weir 31 from the other end of the ejection
container 1.

[0186]The solution storage unit 12 is a container-shaped member which
temporarily stores the solution 11 supplied to the ejection container 1,
and serves as a receiving unit 13 for receiving the solution 11
overflowed the ejection container 1. The solution storage unit 12 is
provided in the vicinity of the other end of the ejection container 1,
that is, in the vicinity of the weir 31, and includes an opening for
directly receiving the solution 11 overflowed over the weir 31. Further,
in the present embodiment, the solution storage unit 12 has a cylindrical
shape having a diameter larger than that of the ejection container 1, and
is provided coaxially to the ejection container 1. The solution storage
unit 12 is provided such that its top end overlaps the other end of the
ejection container 1. With this, it is possible to collect the solution
11 overflowed not only to the bottom, but also to the top or side due to
rotation of the ejection container 1. Furthermore, the base end of the
solution storage unit 12 is closed except the hole into which the rotary
shaft 32 is pivotally inserted.

[0187]The transporting unit 215 is an apparatus for transporting the
solution 11 from the solution storage unit 12 to the ejection container
1. The transporting unit 215 includes a transporting pump 15 for pumping
up the solution 11 from the solution storage unit 12, and a solution
supply tube 10 for guiding the solution 11 to the ejection container 1.
The transporting unit 215 may be a pump which constantly keeps
transporting a predetermined amount of the solution 11; however, it may
be a pump which includes a control unit which is capable of controlling
the transporting amount per unit time in accordance with the storage
amount of the solution 11 in the solution storage unit 12. Further, kinds
of the transporting pump 15 are not particularly limited; and thus, any
pumps such as a gear pump, tube pump or the like, can be used. Note that
use of the tube pump is preferable, since it facilitates conducting the
maintenance.

[0188]The windshield case 216 is a cylindrical box which prevents
evaporation of the solution 11 stored in the solution storage unit 12
from being accelerated by the gas flow generated by the aforementioned
gas flow generating unit 38, and also prevents the solution 11 flowing
through the transporting unit 215 from being influenced by the gas flow.
Such structure is particularly preferable in the case where the gas flow
has high temperature due to heating, since the solution 11 in the
transporting unit 215 can be protected from the high-temperature gas
flow. The windshield case 216 has a tapered portion at one end for
reducing resistance between the gas flow and the windshield case 216, so
as to avoid disturbance of the gas flow as much as possible. Further, the
windshield case 216 has a diameter larger than that of the ejection
container 1, which prevents the gas flow from passing through the
vicinity of the ejection holes 3. Accordingly, the solution 11 travels a
predetermined distance from the ejection holes 3, and then hits the gas
flow. As a result, the direction of travel of the solution 11 changes,
which reduces the possibility of accelerating evaporation of the solution
11 in the vicinity of the ejection holes 3. Consequently, it is possible
to avoid clogging of the ejection holes 3 caused by the solution 11 whose
viscosity is increased under the influence of the gas flow in the
vicinity of the ejection holes 3 or by the nanofibers f produced
immediately after the ejection.

[0189]The solution supplying unit 204 is a unit for directly supplying the
solution 11 to the solution storage unit 12, and includes a solution
supply source 241, an adjusting valve 242 for adjusting the supply amount
of the solution 11, a supply pump 243, and a supply tube 244 for guiding
the solution 11. The solution supply source 241 is a tank for storing the
solution 11. Further, the supply tube 244 passes through inside the
support body 262 which supports the ejection unit 201 (see FIG. 14).

[0190]The rotary shaft 32 is a shaft which has a rod shape and transmits
drive force for rotating the ejection container 1 from the motor 9. The
rotary shaft 32 is inserted into the ejection container 1 through the
other end of the ejection container 1, and is connected to the closed
section of the one end of the ejection container 1.

[0191]The motor 9 is an apparatus which applies rotation drive force to
the ejection container 1 via the rotary shaft 32 for ejecting the
solution 11 through the ejection holes 3 by the centrifugal force. Note
that because of the bores of the ejection holes 3 and the like, it is
preferable that the number of rotation of the ejection container 1 be set
within a range of not less than a few rpm to not more than 10000 rpm.
When the ejection container 1 is directly driven by the motor 9 as in the
present embodiment, the number of rotation of the motor 9 corresponds to
the number of rotation of the ejection container 1.

[0192]The charging unit 202 is an apparatus which chares the solution 11
by applying an electric charge to the solution 11, and includes an
induction electrode 221, a charge power source 16 and a grounding unit
223.

[0193]The induction electrode 221 is a member for inducing charges on the
ejection container 1 which is provided in the vicinity of the induction
electrode 221 and is grounded, by having a voltage higher than ground (by
having a lower voltage in the case where the charge power source applies
a negative voltage). The induction electrode 221 is an annular member
provided so as to surround the tip of the ejection container 1.
Accordingly, when a positive potential is applied to the induction
electrode 221, a negative charge is induced on the ejection container 1,
which results in applying a negative charge to the solution 11. On the
other hand, when a negative potential is applied to the induction
electrode 221, a positive charge is induced on the ejection container 1,
thereby applying a positive charge to the solution 11. Further, the
induction electrode 221 also serves as a guiding body 206 which guides
gas flow from the gas flow generating unit 38.

[0194]The induction electrode 221 needs to be larger than the ejection
container 1 in size. It is preferable that the diameter be set in the
range from not less than 200 mm to not more than 800 mm. The shape of the
induction electrode 221 is not limited to an annular shape, but the
induction electrode 221 may be a polygonal shaped annular member. The
induction electrode 221 only needs to be provided at a certain distance
so as to surround the ejection container 1. The induction electrode 221
may be an annular metal wire or the like which surrounds the ejection
container 1.

[0195]The charge power source 16 is a power source which can apply a high
voltage to the induction electrode 221. The charge power source 16 is a
DC power source, and an apparatus which can change voltage to be applied
to the induction electrode 221 (with ground as a reference potential) or
its polarity.

[0196]Preferable voltage to be applied by the charge power source 16 to
the induction electrode 221 is set within the range from not less than 10
KV to not more than 200 KV. Especially, the electric field strength
between the ejection container 1 and the induction electrode 221 is
important; and thus, it is preferable to set a voltage to be applied or
to arrange the induction electrode 221 such that the electric field
strength is 1 KV/cm or more.

[0197]The grounding unit 223 is a member which is electrically connected
to the ejection container 1 and maintains the ground potential of the
ejection container 1, and serves as a ground. One end of the grounding
unit 223 serves as a brush so that electric connection state can be
maintained even when the ejection container 1 is in a rotating state. The
other end is connected to the ground.

[0198]As in the present embodiment, by utilizing the induction method to
the charging unit 202, an electric charge can be applied to the solution
11 while keeping the ground potential of the ejection container 1. When
the ejection container 1 is in the ground potential state, it is not
necessary that members, such as the rotary shaft 32, the motor 9, and the
solution storage unit 12 which are connected to the ejection container 1,
are electrically insulated from the ejection container 1. This allows a
simple structure of the ejection unit 201.

[0199]Note that it may be that as a charging unit, the ejection container
1 is directly connected to a power source, and an electric charge is
applied to the solution 11 while maintaining the high voltage of the
ejection container 1. Further, it may be such that: the ejection
container 1 is formed of insulating materials; an electrode which
directly contacts the solution 11 stored in the ejection container 1, is
provided inside the ejection container 1; and an electric charge is
applied to the solution 11 by the electrode.

[0200]The gas flow generating unit 38 is an apparatus which generates gas
flow for changing the direction of travel of the solution 11 ejected from
the ejection container 1 into the desired direction of deposition of the
nanofibers f. The gas flow generating unit 38 used in the present
embodiment is a blower including an axial flow fan which forcibly blows
surrounding atmosphere (air). The gas flow generating unit 38 is provided
at the rear side of the motor 9 which rotates the ejection container 1,
and generates gas flow directed the tip of the ejection container 1 from
the direction of the motor 9. The gas flow generating unit 38 is capable
of generating force which changes, into the axial direction of the
ejection container 1, the direction of the solution 11 radically ejected
by the centrifugal force from the ejection container 1.

[0201]The gas flow generating unit 38 may be made of other types of
blowers, such as a sirocco fan. Further, the gas flow generating unit 38
may be a gas flow generating unit which changes the direction of the
ejected solution 11 by introducing high pressure gas. In addition, the
gas flow generating unit 38 may be a gas flow generating unit which
generates gas flow to the inside of the guiding body 206 by the
attracting unit 102, an aforementioned second gas flow generating unit
232, or the like. In this case, the gas flow generating unit 38 does not
include an apparatus for actively generating gas flow; however, in the
case of the present invention, it is considered that the gas flow
generating unit 38 is included since gas flow is generated at a place
close to the ejection container 1. Further, by attracting using the
attracting unit 102 in a state where the gas flow generating unit 38 is
not included, gas flow is generated inside the guiding body 206. This
also be considered that the gas flow generating unit 38 is included. In
FIG. 13, gas flows are indicated by arrows.

[0202]The guiding body 206 is an air channel having a function to guide
gas flow generated by the gas flow generating unit 38 into a
predetermined direction.

[0203]The heating unit 205 is a heating source which heats gas (safe gas)
forming the gas flow generated by the gas flow generating unit 38. In the
present embodiment, the heating unit 205 is an annular heater provided on
the air path formed by the guiding body 206, and is capable of heating
gas passes through the heating unit 205. By heating gas flow using the
heating unit 205, evaporation of the solution 11 ejected into the space
is accelerated, thereby effectively producing the nanofibers f.

[0204]Further, the nanofiber discharging apparatus 200 includes a solution
amount detecting unit 291 and a supply amount control unit 292.

[0205]The solution amount detecting unit 291 is an apparatus which detects
the storage amount of the solution 11 stored in the solution storage unit
12. The solution amount detecting unit 291 shown in FIG. 13 includes a
float 293 floating in the solution 11. The solution amount detecting unit
291 is an apparatus which detects upper limit height and the lower limit
height of the solution 11 stored in the solution storage unit 12 by the
liquid surface sensor 35 which detects, by two limit switches (not
shown), up and down movement of the float. As described, in the case
where the shape of the solution storage unit 12 is known or can be
measured, it is only necessary to simply detect the height of the liquid
surface. The solution amount detecting unit 291 is not limited to the
method above, but it may be a solution amount detecting unit which
detects the height of the surface of the solution 11 linearly. Further,
since the solution 11 is charged, it is preferable that the liquid amount
detecting unit mechanically detects the liquid amount as the described
floating type does.

[0206]The supply amount control unit 292 is an apparatus which controls an
adjusting valve 242 in the solution supplying unit 204 based on the
detection result of the solution amount detecting unit 291 such that the
storage amount of the solution 11 is within a predetermined range. In the
case where two detection results, which are the upper limit and the lower
limit of the liquid surface of the solution 11, are transmitted from the
solution amount detecting unit 291, the supply amount control unit 292
starts supplying the solution 11 when the liquid surface reaches the
lower limit, and stops supplying the solution 11 when reaching the upper
limit. Further, in the case where the detection result of the height of
the liquid surface of the solution 11 is transmitted linearly, it may be
that the supply amount control unit 292 performs calculation based on the
height of the liquid surface and the supply amount, and adjusts opening
of the adjusting valve 242 so that a constant liquid surface can be kept
as much as possible. Note that examples of such control include a PID
control.

[0207]Note that the supply amount control unit 292 may control supply
amount by directly controlling the supply pump 243, instead of
controlling the adjusting valve 242.

[0208]Accordingly, the solution 11 stored in the ejection container 1 can
be kept to be an approximately constant amount. Therefore, condition of
the solution 11 ejected through the ejection holes 3 can be stabilized,
thereby stabilizing the quality of the produced nanofibers f. Further,
the amount of the solution 11 stored in the solution storage unit 12
provided near the ejection container 1 is kept within a predetermined
range, or at a predetermined amount. Therefore, it is possible to
continuously supply the solution 11 stably without adversely affecting
the amount of the solution 11 in the ejection container 1. In addition to
those advantageous effects, circulation pathway of the solution 11,
provided for keeping the constant amount of the solution 11 in the
ejection container 1, has such a length that it cannot be shortened any
further. Therefore, it prevents, as much as possible, degradation of the
solution 11 caused by circulation of the solution 11. As a result, it is
possible to stabilize quality of the produced nanofibers at a high level.

[0209]More particularly, the ejection container 1 often rotates at a high
speed of 1000 rpm or more. In the case where the ejection container 1
contains a large amount of solution 11, uneven rotation or shift of the
rotary shaft causes the ejection container 1 to rotate improperly, which
largely contributes to the breakdown of the apparatus.

[0210]FIG. 14 is a perspective view of an appearance of an ejection unit.

[0211]The support body 262 is a member for supporting the ejection unit
201, and provided between the ejection holes 3 and the gas flow
generating unit 38. The support body 262 has a thickness smaller than the
diameter of the ejection container 1 in a vertical direction with respect
to the direction of flow of the gas flow generated by the gas flow
generating unit 38. The support body 262 has a long shape extending along
the direction of gas flow. Such shape is for strongly supporting the
ejection unit 201 while preventing disturbance of the gas flow as much as
possible. Further, the support body 262 has an end that is at the
upstream side of the gas flow, and the other end that is at the
downstream side of the gas flow, which both have a streamline shape.
Having the streamline shape in such a manner further prevents the
disturbance of the gas flow.

[0212]Further, the support body 262 includes a supply tube 244 for
supplying the solution 11 to the solution storage unit 12 inside. The
support body 262 further includes an insertion hole 283 into which a
conductive wire or the like for supplying electric power to the motor 9
is inserted. By including the supply tube 244, and the insertion hole 283
inside the support body 262 in such a manner, it is possible to prevent
the disturbance of the gas flow generated by the gas flow generating unit
38.

[0213]Further, the support body 262 includes a bearing 5 at its bottom end
edge. The bearing 5 pivotally supports the ejection container 1 at the
bottom end edge f the support body 262.

[0215]The air channel 265 is a member which forms an air channel for
guiding the solution 11 or the nanofibers f discharged from the solution
discharging unit 290 such that they pass through a predetermined travel
path. The air channel 265 includes, at its base end, an inlet opening for
receiving, together with the gas flow generated by the gas flow
generating unit 38, the solution 11 or the nanofibers f discharged from
the solution discharging unit 290. Following the inlet opening, the air
channel 265 includes an electrostatic explosion area where a space is
formed in which the solution 11 undergoes sufficient electrostatic
explosions so that the nanofibers f are produced. Further, following the
electrostatic explosion area, the air channel 265 includes a charge
neutralization area where a space is formed in which the charges of the
nanofibers f, produced by the electrostatic explosion and still being in
charged states, are neutralized. The charge neutralization area may have
a length long enough for the charges of the nanofibers f to be naturally
neutralized. Further, the charge neutralization area may include a charge
neutralizer 207 for forcibly neutralize the charges of the nanofibers f.

[0216]The charge neutralizer 207 is an apparatus which forcibly
neutralizes the charged nanofibers f, and discharges, into a space, ions
or particles having a polarity opposite to that of the charged nanofibers
f. More specifically, the charge neutralizer 207 may utilize any types of
methods, such as a corona discharge type, voltage applying type, AC type,
stationary DC type, pulsed DC type, self discharge type, soft x-ray type,
ultraviolet ray type, and radiation type.

[0217]Following the charge neutralization area, the air channel 265
includes a narrowing area whose bore (area) gradually narrows from
upstream side to downstream side of the gas flow. The narrowing area has
a tapered shape which improves density of the nanofibers f that are
present in the space. Each of the upstream side and the downstream side
of the gas flow in the narrowing area includes a gas flow inlet 233. The
gas flow inlet 233 is connected to the second gas flow generating unit
232, and is an opening for guiding rapid gas flow into the air channel
265. Each of the gas flow inlets 233 is provided toward a direction that
the gas flow can be ejected from the larger bore side to the smaller bore
side of the narrowing area.

[0218]The second gas flow generating unit 232 is an apparatus which
generates gas flow by introducing high pressure gas into the air channel
265. More specifically, an example of the second gas flow generating unit
232 is an apparatus which includes a tank (cylinder) which can store high
pressure gas, a pump for forcibly introducing gas into the tank, and a
gas introducing unit having a valve for adjusting pressure of high
pressure gas in the tank.

[0219]Note that the gas supplied by the second gas flow generating unit
232 may be air, but preferably safe gas which has oxygen content ratio
lower than that of air. This is to avoid explosion due to solvents
evaporated from the solution 11. Examples of the safe gas include low
oxygen concentration gas, in which a certain amount of oxygen is removed
from air by using a resin film (hollow fiber membrane), and superheated
steam. The description here does not exclude the use of high purity gas
which hardly contains oxygen, but, for example, high purity nitrogen
sealed in a cylinder in the form of liquid or gas, or carbon dioxide
supplied from dry ice may also be used.

[0220]Further, a heating unit may be provided for heating gas flow
generated by the second gas flow generating unit 232.

[0221]Next, outlines of a method for producing the nanofibers f and a
method for producing nonwoven fabric will be described.

[0222]First, the gas flow generating unit 38 generates gas flow into the
solution discharging unit 290 and the guiding body 206. At the same time,
the attracting unit 102 attracts the gas flow from a position which is
farther downstream than the collector 20.

[0223]Next, the solution 11 is supplied into the solution storage unit 12,
and is transported from the solution storage unit 12 to the ejection
container 1. Next, the solution 11 stored in the ejection container 1 is
electrically charged by the charge power source 16, and the ejection
container 1 is rotated by the motor 9, so that the charged solution 11 is
ejected through the ejection holes 3 by the centrifugal force.

[0224]The ejected solution 11 is changed its direction of travel by the
gas flow. As a result, the ejection holes 3 can be arranged vertically or
substantially vertically to the deposition surface of the nanofibers f,
thereby ejecting a large amount of solution 11 into a certain space.
Further, since the gas flow is heated, evaporation of solvents is
accelerated, which results in acceleration of electrostatic explosion. As
a result, it is possible to effectively produce the nanofibers f.

[0225]Here, the windshield case 216 prevents the gas flow from reaching
the ejection holes 3 or near the ejection holes 3. This makes a state
where evaporation of the solvents included in the solution 11 is not
easily accelerated near the ejection holes 3. As a result, narrowing and
blocking the ejection holes 3 by the solute are prevented. Therefore, it
is possible to suppress reduction of the ejection amount as much as
possible, even if the solution 11 is continuously ejected from the
ejection container for a long period of time. More specifically, it is
possible to maintain the concentration of the solution 11 or the
nanofibers f in the air at a high state for a long period of time.

[0226]Then, the produced nanofibers f are transported in the air channel
265 with the gas flow, and reaches the collector 20 while being in the
high density state. The collector 20 serves as a filter, since the gas
flow is attracted by the attracting unit 102 from the rear side
(downstream side). The collector 20 separates the nanofibers f and the
gas flow, and collects only the nanofibers f while depositing the
nanofibers f thereon. The collector 20 on which the nanofibers f are
deposited is moved at a predetermined moving speed by the winding of the
transporting unit 104. The nanofibers f deposited on the collector 20 is
moved together with the collector 20 while forming a nonwoven fabric, and
wound by the transporting unit 104.

[0227]Accordingly, the nanofiber producing apparatus 100 according to the
present embodiment is capable of stably producing high quality nanofibers
f. In addition, it is possible to make a state where the concentration of
the solution 11 or the produced nanofibers f in the air is high and even,
and also to collect the nanofibers f in such a high concentration state.
Accordingly, it is possible to stably collect the nanofibers f in a thick
and long non-woven fabric state with high quality.

[0228]Note that in the present embodiment, the support body 262 supports
the ejection unit 201 in a hanging state; however, the present invention
is not limited thereto. For example, it may be that the support body 262
is attached to the floor or the like, and supports the ejection unit 201
such that the ejection unit 201 is placed thereon.

[0229]Further, in the above embodiment, the supply amount of the solution
11 is controlled by detecting the solution storage amount of the solution
storage unit 12; however, it may be that the amount of the solution 11
ejected from the ejection holes 3 is predicted, and the amount of the
solution 11 which is approximately same amount of the predicted
consumption amount is continuously supplied from the solution supply unit
204.

[0230]FIG. 15 is a cross-sectional view of a variation of the ejection
unit.

[0231]As shown in FIG. 15, the cylindrical ejection container 1 having a
closed one end, does not have a weir on its other end. Therefore, the
diameter of the inside of the ejection container 1 is same from one end
to the other end. Further, the solution supply tube 10 of the
transporting unit 215 is inserted into the ejection container 1, and has
a discharging portion 10a at the tip. The discharging portion 10a is
positioned near the closed end of the ejection container 1.

[0232]Therefore, in the ejection container 1 according to the present
variation 1, the solution 11 is supplied to the innermost portion of the
ejection container 1. Then, the solution 11 overflows along the inner
circumferential surface of the ejection container 1 through the opening
(some of the solution 11 is discharged through the ejection holes 3).
Therefore, resistance due to the weir does not occur when the solution 11
overflows.

[0233]It is preferable that such structure is applied, for example, to the
solution 11 having a high viscosity. This is because, in the case of the
high-viscosity solution 11, it is possible to form a layer of the
solution 11 with a desired thickness on the inner circumferential surface
of the ejection container 1 due to the centrifugal force caused by
rotation of the ejection container 1, even without the weir.

[0234]FIG. 16 is a cross-sectional view of another variation of the
ejection unit.

[0235]As shown in FIG. 16, the ejection container 1 is pivotally attached
to the solution supply tube 10 which is fixedly arranged. Further, the
solution supply tube 10 includes a plurality of discharging portions 10a.

[0236]With the above structure, it is possible to supply the solution 11
evenly in the longitudinal direction of the ejection container 1. Such a
structure is preferable especially when the ejection container 1 is long.

INDUSTRIAL APPLICABILITY

[0237]According to a nanofiber producing method and apparatus of the
present invention, the amount of solution in an ejection container can be
always maintained to be constant by allowing the amount of the solution
exceeding a predetermined amount in the ejection container to overflow,
and simply supplying a sufficient amount of the solution. Therefore,
centrifugal force acts on the solution discharged through ejection holes
of the ejection container can be made constant, and uniform nanofibers
can be always produced. As a result, the method and apparatus can be
preferably used for producing, with high productivity, high quality
nanofibers that are preferably applied to a filter, a separator for use
in a battery, a polymer electrolyte membrane or an electrode for use in a
fuel cell, or the like.